Steroid hormones regulate cell proliferation. Sex steroid stimulate proliferation of epithelial cells of the breast, uterus, vagina, and prostate: whereas glucocorticoids inhibit proliferation of cells of connective tissue origin including keratinocytes, fibroblasts, and lymphocytes. The mitotic effects of steroids appear to be due to regulation of genes whose products are required for progression through the G1 phase of the cell cycle. This proposal focus upon glucocorticoid inhibition of lymphoid cell proliferation, which is associated with Go arrest. Both normal and malignant lymphoid cells undergo Go arrest ina the presence of glucocorticoids, and steroids of this class are routinely used in the treatment of both malignant and nonmalignant lymphoproliferative disorders. Malignant transformation of lymphoid cells may be associated with expression of cell cycle progression factors that are not regulated by glucocorticoids (e.g. Bcll) Our long term objective is to identify the genes that account for Go arrest and to elucidate the molecular mechanisms that account for their regulation by glucocorticoids. We have shown that there are two target genes that account for Go arrest. c-myc and the cyclin D3 gene (CcnD3). This project focuses upon the mechanisms that underlie glucocorticoid inhibition of cyclin D3 expression. The effect is rapid and large: CcnD3 mRNA abundance decreases by 80-90% within 4-5 hr after addition of dexamethasone to mid log phase T lymphoma cells. There is no change in CcnD3 transcription during this time frame. We have determined the CcnD3 mRNA is very stable in control cells, with a half life in excess of 8 hr. Within 2 hr after addition of dexamethasone, the T12 of degradation decreases to 60-90 min. This response requires de novo RNA synthesis. Moreover, deletion of the 3' untranslated region of CcnD3 mRNA blocks the response. We propose that glucocorticoids activate a factor that interacts, directly or indirectly, with the 3' untranslated region of CcnD3 mRNA and increases the rate of degradation of that messenger. Eight specific aims are proposed to test predictions that emmanate from this working hypothesis.